1. Field of the Invention
The present invention relates to control strategies for engines, and more particularly, to control strategies for rapidly heating an emission control device.
2. Background of the Invention
Emission control devices, such as catalytic converters, can reduce emissions generated by engine combustion. However, the effectiveness of such emission control devices varies with operating temperature. For example, the efficiency of an emission control device may be much greater at higher temperatures than it is at lower temperature. Typically, a “light-off” temperature is used to signify a certain temperature above which a prescribed efficiency is achieved.
In vehicle emission control systems, measures are taken to rapidly heat emission control devices after starting of a vehicle, so that tail pipe emissions can be minimized. In particular, as described in U.S. Pat. No. 5,479,745, an ignition timing signal is retarded during cold engine operation to rapidly warm a catalyst converter coupled to the engine exhaust. By retarding ignition timing, additional heat is provided to the exhaust, thereby providing more heat to increase catalyst temperature.
The inventor herein has recognized a problem with approaches such as the one described above. In particular, as emission requirements are continually lowered, attempts are made to add more and more heat to the catalyst to obtain faster and faster catalyst light-off. In particular, the inventor herein has found that as ignition timing is retarded further (to obtain more heat), engine combustion becomes inconsistent. This inconsistency in combustion (due to the very late ignition timing) leads to vibration during idle conditions, which can degrade customer satisfaction. Further, such vibration can also lead to degraded idle speed control.
While the inventor herein has found that increasing idle speed can reduce the above problems, such action has still other problems. For example, operating at a higher engine speed requires additional fuel, which can degrade fuel economy. Also, higher idle speed can also degrade customer satisfaction since additional noise may be generated.
Another measure taken to rapidly heat an emission control device is to operate some cylinders combusting a mixture lean of stoichiometry and others a mixture rich of stoichiometry. When the lean and rich combustion gases mix in the exhaust, the un-burnt reactants from the rich gases react with excess oxygen in the lean gases, thereby generating heat. Such a system is described in U.S. Pat. No. 6,189,316.
However, the inventor herein has recognized problems with this approach also. In particular, the amount of heat generated is typically proportional to the amount of excess oxygen and reactants in the exhaust. To increase heat generated, the lean bank can be run even leaner, and the rich bank even richer. However, the degree of leanness in the lean bank is limited by combustion inconsistency, just as the amount of ignition retard is limited. Thus, attempting to run too lean leads to similar problems as described above, such as engine vibration.